The expense of oil and gas exploration and development is heavily dependent on the efficiency of the drilling system utilized to drill exploration wells to identify deposit sites and production wells to exploit the deposit sites once identified. It is therefore highly desirable to maximize drilling efficiency in terms of the effective overall speed of the drilling system. To this end, there have been many attempts to improve drilling efficiency by finding methods of assisting the direct drilling action of a drill bit utilized in the cutting of the substrate being drilled.
One type of conventional drilling system is referred to as rotary percussion drilling. Rotary percussion drilling is based on providing repeated percussion impacts or blows to a drill bit either through the drill string--commonly referred to as "top hammer" or at the location of the drill bit--commonly referred to as "down the hole"--as the drill bit is rotated and a thrust load or feed force is applied to the drill string. Flushing must also be provided in the rotary percussion drilling system to remove drill cuttings from the path of the bit as the bit is rotated and advanced under the force of the percussion impact, and is usually accomplished by providing a jet of water through an opening in the drill string. In a top hammer system, a hammer device is typically driven by compressed air to strike a shank of the drill string, which in turns imparts a shock wave to the drill string that travels to the bit. As drilling depths increase, however, it is increasing difficult to generate a shock wave of sufficient energy at the location of the bit.
In order to overcome the deficiencies of a top hammer system, a down the hole system drilling system was developed to apply a force directly to the drill bit at the bottom of the drill string. The down the hole drilling system incorporates a hydraulic or pneumatic piston hammer in the drill string at a location directly adjacent to the drill bit. Accordingly, the problem of dissipation of the shock wave experienced in top hammer drilling systems is overcome. However, as down the hole hammers are mechanical devices with moving components of some complexity, they are inevitably subject to wear or breakage and require periodic removal from the drill string for maintenance and repair, thereby slowing the overall drilling operation.
Due to the problems associated with both top hammer and down the hole drilling systems, alternative methods have been sought for improving drilling efficiency that would not rely on the mechanical transmission of a shock wave to a conventional roller bits. One such alternative method referred to as abrasive waterjet drilling utilized the additional of high pressure fluid jets to a conventional roller head to assist in breaking up or cutting the rock. For example, a drilling fluid including an abrasive such as a steel shot is forced from the fluid jets at high pressure to impact the rock surrounding the drill bit. Tests conducted using abrasive waterjet drilling showed that drilling rates could be significant improved over conventional rotary percussion drilling techniques. Drilling rates could only be maximized, however, when pressure and abrasive concentrations could be optimized at the bit. Accordingly, like rotary percussion drilling discussed above, abrasive waterjet drilling also exhibited a significant problem, namely, as the drill string became longer it became increasingly difficult to maintain the constant high pressures necessary over the entire length of the drill string to generate sufficient energy at the jets. Further, changes in pressure along the length of the drill string, due to temporary blockages at the nozzle or a change in fluid density, resulted in changes in the length of the drill string due to pressure differentials, which in turn caused overloading of the drill bit or lifting of the drill bit off the rock so drilling could not be maintained. In addition, problems were encountered in finding an effective method of introducing an abrasive into the drill fluid and recycling the abrasive for subsequent use. Accordingly, although the possibility of significant improvements in drilling rates were experimentally shown, further development of abrasive waterjet drilling was essentially abandoned.
In view of the above, it is an object of the invention to provide a drilling method and apparatus having improved efficiency that does not exhibit the drawbacks of conventional rotary percussion drilling and abrasive waterjet drilling discussed above.